Combination plastic and gas injection nozzle assembly and sequential method of operation

ABSTRACT

A combination plastic and gas injection nozzle assembly and method of operation in which the nozzle assembly includes a plastic resin passage, a closure member for the plastic resin passage, a gas flow passage, and a closure member for the gas flow passage all coaxial with respect to one another. Actuating cylinders are provided for moving both closure members between open and closed positions. The actuating cylinders being positioned coaxial with the other elements to eliminate off-set loading of the nozzle components. A sequential molding operation is disclosed in which a plurality of mold cavities defined by a single mold assembly are injected by one or more nozzles with plastic and a gas to form hollow spaces in the molded article. The cavities are injected with plastic one at a time to reduce the required mold clamping forces and enable independent control of plastic injection into each cavity. After partially filling each mold cavity, the nozzles are used to inject pressurized gas into the molten resin forcing the resin against the surface of the mold cavities and forming hollow spaces within the molded articles.

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates to a plastic injection nozzle assembly andin particular to a nozzle assembly for use in injecting both plastic anda gas into a mold cavity. Such a nozzle assembly can be used forinjection molding of thermal plastic parts with hollow walls, hollowribs or hollow parts with solid webs connecting the walls together.

Injection molding of hollow parts is accomplished by first injectinginto a mold cavity a predetermined quantity of molten plastic which isless than the volume of the mold cavity. After the plastic is injected,a gas is injected into the molten plastic forcing the plastic throughthe mold cavity and holding the plastic against the wall of the moldcavity while the plastic cools. This aids the external surface of theplastic material in assuming the precise shape dictated by the moldsurface. The gas pressure also assists in filling the narrow portions ofthe mold cavity with plastic which is often difficult to accomplish andcan only be accomplished with high injection pressures in a solidplastic injection molding. The use of the gas pressure will reduce thetendency of the plastic to shrink away from the mold walls duringcooling. In solid injection molding the plastic is kept under highpressure to prevent shrinkage during cooling by maintaining the plasticinjection pressure. However, this produces unwanted stress in the moldedpart as well as significant stress in the injection molding machinery.

In gas injection molding, articles are produced by injecting moltenplastic into the mold cavity and charging a body of pressurized gastherein to form a hollow portion in the thermal plastic material. Thegas may be nitrogen, air or other gas that will not react with thepolymer being injected. Pressure is maintained on the gas in the hollowspace within the molded object until the plastic material in the moldcavity has set. Thereafter, the pressurized gas is released from themolded part hollow area and the molded part can be taken out of the moldcavity.

Several nozzle assemblies have been proposed for both gas and plasticinjection. These nozzles, however, suffer from one or more of thefollowing deficiencies. Some nozzles require that the nozzle assembly bephysically removed from the mold in order to vent the mold interior toexhaust the gas from the molded part interior. This is particularlydisadvantageous when multiple nozzles are required to mold large parts.Some nozzles are prone to plastic plugging of the gas passage near thetip of the nozzle assembly. In order to prevent this plugging, the gaspassages are often made extremely small in size which decreases the gasflow rate into and out of the mold cavity. Another disadvantage is thatthe actuators used to move various internal components of the nozzleassembly are often off-set to one side of the nozzle center line. Thiscan create uneven loading of the moving components resulting in bindingof these components, increasing wear and nozzle maintenance.

It is an objective of the present invention to provide a combination gasand plastic injection nozzle assembly that overcomes the above mentioneddeficiencies.

The combination gas and plastic injection nozzle of the presentinvention includes a tubular nozzle body that is tapered at one end toform a nozzle tip. The nozzle body forms an orifice at the tip throughwhich the molten plastic is injected into the mold. The opposite end ofthe nozzle body is coupled to a conventional injection molding machinefor receiving molten plastic resin. A hollow closure rod extendslongitudinally through the nozzle body and serves as a plug that ismovable into the nozzle body orifice to close the orifice, stopping theflow of plastic. The annular space between the exterior of the hollowrod and the nozzle body forms a flow passage for the molten plastic. Theorifice is opened by retracting the nozzle rod into the nozzle body.

The interior of the hollow closure rod forms a passage for the injectiongas. The end of the nozzle rod forms a small orifice through which thegas is injected into the mold after the nozzle rod has closed the nozzletip to plastic. A second hollow rod, a gas rod, extends through thenozzle rod to supply the injection gas to the orifice at the end of thenozzle rod. The gas rod ends in a closed tip which is seated against aninterior engagement surface of the nozzle rod to close the gas orifice.The gas rod is longitudinally movable away from the seat to initiate theflow of gas. Openings in the gas rod before the closed tip enable thegas to flow from the gas rod into the gas passage in the nozzle rod.

Both the nozzle rod and the gas rod are moved by actuating cylindersthat are positioned in line with the axis of the nozzle body so as toavoid an off-set load of either rod as they are moved.

It is a further object of the present invention to provide an improvedprocess utilizing a mold with multiple cavities and at least one nozzleassembly for each mold cavity.

The injection nozzle assemblies are particularly useful in a multiplenozzle injection molding process. Injection molding process efficienciescan be improved by providing a mold with multiple cavities to producemultiple articles with a single closing and opening of the mold. Byusing multiple nozzles with at least one nozzle per cavity within a moldassembly, a sequential process can be used in which the cavities arefilled with plastic one at a time rather than all cavities being filledsimultaneously. Such a sequential process has numerous advantageous overa simultaneous process using one nozzle connected to several moldcavities by a runner system. By separately injecting the resin into eachcavity, the quantity of plastic to each cavity and the flow rate can becontrolled individually for each cavity. Likewise, the subsequent gasinjection into each cavity can also be independently controlled. Afurther advantage is that the mold clamping load can be significantlyreduced since only one cavity is subjected to the plastic injectionpressure at any given time. This enables lighter weight mold componentsto be used and also decreases the process time required to build up thenecessary mold clamping force.

Further objects, features and advantages of the invention will becomeapparent from a consideration of the following description and theappended claims when taken in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are partial sectional and partial elevational views ofthe nozzle assembly with each figure illustrating half of the nozzleassembly;

FIG. 2 is a sectional view of a portion of the nozzle assembly and amold showing plastic being injected into the mold cavity;

FIG. 3 is a sectional view similar to FIG. 2 showing gas being injectedinto the plastic in the mold cavity;

FIG. 4 is a schematic illustration of the control systems for the nozzleassembly;

FIG. 5 is a partial sectional and partial elevational view of a moldassembly with the nozzle assemblies of the present invention; and

FIG. 6 is a sectional view of a portion of the nozzle assembly showingan alternative embodiment.

DETAILED DESCRIPTION OF THE INVENTION

The combination gas and plastic injection nozzle assembly of the presentinvention is shown in FIGS. 1A and 1B and designated generally at 20.Nozzle assembly 20 is constructed upon a nozzle block 22 coupled to athermal plastic resin supply manifold 24. Inlet 26 of the nozzle blockreceives the molten plastic resin from passage 28 in the manifold.Manifold 24 is in turn coupled to a conventional extrusion injectionunit such as a reciprocating screw or a screw and plunger type unit (notshown). Threaded into the nozzle block and retained by a locking collar30 and bolts 32 is a tubular nozzle body 34. The surface 36 of thenozzle body is tapered adjacent the distal end 38 of the body, forming anozzle tip 40. The inner surface of the nozzle body at 42 is likewisetapered radially inwardly toward the end 38 and forms an orifice 44 atthe tip 40.

A rod 46 extends longitudinally through the nozzle body 34 and isgenerally centered within the nozzle body about the longitudinal axis48. Rod 46, referred to as a nozzle rod or a closure rod, islongitudinally movable within nozzle body 34 as will be described below.In FIG. 1, the nozzle rod 46 is shown in a closed position inserted intothe orifice 44 where it acts as a plug to close the orifice and thenozzle assembly to the flow of plastic. Nozzle rod 46 is guided bybushing 50 within the nozzle block. Bushing 50 also prevents pressurizedplastic from leaking from the nozzle block 22.

The nozzle rod 46 extends beyond bushing 50 and is coupled to the pistonrod 52 of the nozzle rod actuating cylinder 54 through the threadedcoupling 56. The cylinder 54 is mounted to the nozzle block 22 by nutand bolt assemblies 58. Sleeves 60, between the end plate 62 of cylinder54 and the nozzle block position the cylinder 54 relative to the nozzleblock. The actuating cylinder 54 is in line with the axis 48 of thenozzle assembly.

When actuated, the cylinder 54 retracts the nozzle rod 46 from theorifice 44 to the open position shown in FIG. 2 allowing plastic fromthe manifold to flow through the annular plastic flow passage 64 betweenthe nozzle rod 46 and nozzle body 34 and through the orifice 44 into themold. Once the desired volume of plastic resin has been injected intothe mold, the cylinder 34 will again be actuated to move the nozzle rod46 to the closed position shown in FIG. 1, closing the nozzle orifice44. Due to the high plastic resin pressures, actuating cylinder 44 willtypically be a hydraulic cylinder.

The nozzle rod 46 is also hollow, forming a fluid passage 66 near thenozzle tip 40 for a pressurized fluid, typically gas, to be injectedinto the mold. The end of the nozzle rod forms a small diameter orifice68 of a relatively short length. The interior surface of the nozzle rodis tapered at 70 reducing the diameter of the interior passage leadingto orifice 68.

Pressurized gas is provided to the gas passage 66 through a hollow gasrod 72 that extends through the nozzle rod 46 and the piston rod 52. Theend of gas rod 72 at nozzle tip includes a solid tip 74. The gas rod tipis shown in FIG. 1 in the closed position seated against the taperedsurface 70 at orifice 68. The gas rod 72 includes vent openings 76adjacent the tip 74 to allow pressurized gas to travel from the gas rodinto the passage 66 and through orifice 68 into the mold cavity.

The gas rod 72 is longitudinally movable to retract the tip 74 from seat70 to allow passage of the pressurized gas through orifice 68 into themold cavity. FIG. 3 illustrates the gas rod in the open position.

Gas rod 72 is longitudinally moved by gas rod actuating cylinder 78. Thepiston rod 80 of actuating cylinder 78 is coupled to the end of gas rod72 and forms a gas flow passage 82 therethrough and a fitting 84 at itsopposite end for connection with a gas supply.

The gas rod 72 is seated against the nozzle rod tapered surface 70 toclose the orifice 68. This orifice is closed when the nozzle rod ismoved, the gas rod must move the gas rod actuating cylinder 78 ismounted to the piston rod 52 of the nozzle rod actuating cylinder 54 sothat orifice 68 is kept closed. Gas rod actuating cylinder 78, likenozzle rod actuating cylinder 54, is in line with the axis 48 of thenozzle assembly to eliminate off-set loads to the nozzle rods andresulting binding and wear. Guide rods 86, projecting axially from theactuating cylinder 78, extend through the end plate 88 of nozzle rodactuating cylinder 54 to guide the gas rod actuating cylinder, as it ismoved. The nozzle rod 46 inner bore of the end of the nozzle rodopposite tip 40 is of a reduced internal diameter 89 to provide abearing surface for the gas rod.

A stroke limiter 90 is threadably mounted on the piston rod 52 and heldin place by jam nut 92. Stroke limiter 90 is used to adjust the lengthof travel of the nozzle rod 46. The position of the nozzle rod endrelative to the tapered inner surface 42 of the nozzle body can be usedto throttle the flow of the plastic resin through the annular resinpassage 64.

A static seal 94 held in place on the nozzle rod by nut 96 prevents gasfrom escaping from the gas passage 66 through the threaded coupling 56between the nozzle rod 46 and piston rod 52. Likewise, seal 98 and nut100 prevent gas from leaking past the coupling of the piston rod 52 andthe gas rod actuating cylinder 78. The use of a static gas sealseliminates wear of the seal that occurs with dynamic seals. The staticseal thus reduces maintenance and down time of the nozzle assembly.

An alternative embodiment of the nozzle is shown in FIG. 6 whereidentical components are given the same reference numbers as in FIGS. 1Aand 1B while similar but modified components are the same referencenumber followed by the suffix "a". In this embodiment, the gas rod tip74a extends from the nozzle assembly rather than retracts into theassembly to open the gas passage. The tip 74a terminates in afrusto-conical enlargement 75. A portion of the conical surface 77engages the tapered seat 70a of the nozzle rod 46a to close the gaspassage. Actuating cylinder 78 is used to hold the tip 74a in the closedposition.

When the gas is to be injected into the molten resin in the mold, thecylinder 78 is vented to atmosphere on both sides of the piston whilethe gas passage 66 is pressurized with the injection gas. When theplastic pressure force acting on surface 79 at the end of tip 74areduces to a value lower than the gas pressure force acting on theconical surface 77 in the opposite direction, the gas forces will causethe tip to move into the plastic to the open position shown in FIG. 6.This ensures that the gas flow orifice will not open until the plasticpressure is reduced to avoid plastic back flow into the gas orifice.Once the gas passage opens, the cylinder 78 is pressurized to hold thegas rod and tip in the open position until the plastic cools and the gasvented. After this, the cylinder 78 is actuated to close the gaspassage.

FIGS. 2 and 3 illustrate the injection molding operation with nozzleassembly 20 coupled to a mold assembly 102. The mold assembly includes astationary mold half 104 with a tapered aperture 106 sized to receivethe nozzle tip 40. Movable mold half 108 and stationary mold half 104define a mold cavity 110 in communication with the opening 106. Thenozzle rod 46 is retracted in FIG. 2 to open the nozzle body orifice 44allowing a shot of plastic resin 112 to be injected into the moldcavity. The plastic shot is of a smaller volume than the cavity 110.Once the plastic is injected, the nozzle rod 46 is moved to the closedposition shown in FIG. 3 terminating the flow of plastic into or out ofthe mold. After orifice 44 is closed, the gas rod 72 and tip 74 areretracted from orifice 68 and pressurized gas is injected through gasrod 72, vent opening 76, gas passage 66 and orifice 68 into the moldcavity. The pressurized gas forces the plastic resin outward against thewalls of the mold cavity forming a hollow area 114 in the moldedarticle. The gas rod 72 is maintained in the open position and the gaspressure maintained in the mold cavity while the plastic resin iscooled. When the resin is cooled sufficiently that the article can beremoved from the mold, the gas is exhausted from the mold cavity,reducing the pressure in the hollow area 114 to approximatelyatmospheric pressure. After which, the gas rod is returned to the closedposition engaging seat 70 and the mold is opened and the articleremoved.

FIG. 4 illustrates the hydraulic and pneumatic control systems for theactuating cylinders 54 and 78 and for supplying pressurized gas to themold cavity. The control system for actuating cylinder 54 includes adirectional valve 124 and a source of hydraulic pressure 126 including apump, pressure regulating valve and accumulator. The valve 124 is springbiased to the position shown in FIG. 4 in which fluid pressure isnormally provided to the left side of cylinder 54 as viewed in FIG. 1holding the nozzle rod 46 in the closed position. When the nozzle rod isto be retracted to permit plastic flow, the valve is actuated, directingthe hydraulic fluid to the right side of cylinder 54 and draining theleft side of the cylinder to a reservoir.

Pneumatic cylinder 78 is controlled by a directional valve 128 and apressurized gas source 130 including a pump and pressure regulatorvalve. The cylinder 78 is normally pressurized on the left side holdingthe gas rod tip 74 against the tapered seat 70. When valve 128 isactuated, the air flow is directed to the right side of the cylinder,retracting the gas rod to open orifice 68.

The supply of pressurized gas to the mold cavity is regulated bypneumatic control system 132. Pump 134 designates a source of highpressure gas. The pump may be used to fill receiver tanks (not shown)with pressurized gas. The gas may also be supplied by other means suchas a common plant gas supply or a high pressure gas tank. The pump 134is illustrative of one way of supplying a pressurized gas and is not tobe viewed as a limitation.

Pressure regulator 136 reduces the gas pressure to the desired injectionpressure and the gas flow is controlled by valve 138. Solenoid valve 140starts and stops the gas injection. A gas pressure switch 142 shuts offthe gas if mold cavity pressure does not obtain a predetermined value.After gas injection, once the molded part has cooled, valve 140 isreturned to the closed position and an exhaust valve 144 is opened.

Valve 144 can vent the gas to atmosphere or, as shown in FIG. 4, the gascan be vented to a recovery system 146 to recover and reuse thepressurized gas. Recovery system 146 includes an exhaust pump 148,receiver 150, filter and separator 152 and ultimately returns the gas tothe vacuum side of pressurizing pump 134. When using a gas other thanair, the operating cost of the nozzle assembly is reduced if the gas isrecovered for reuse. Furthermore, use of an exhaust pump 148 decreasesthe time necessary to reduce the gas pressure in the molded article to apressure at which the mold can be opened. This reduces the molding cycletime, improving the machine efficiency.

The sequential molding process is described with reference to FIG. 5. Amultiple cavity mold assembly 156 is shown including a stationary platen158 and tie rods 160. Stationary mold halves 162 and 164 are attached tothe stationary platen. Movable mold halves 166 and 168 are carried by amovable platen (not shown). Mold halves 162 and 166 cooperate to definea mold cavity 170 which is fed by a gas and plastic nozzle assembly 172.Mold halves 164 and 168 define a mold cavity 174 which is fed by twonozzle assemblies 176 and 178. Nozzle assemblies 172, 176 and 178 areidentical to nozzle assembly 22 described above.

The molding operation is performed by first closing the mold assembly156. After a sufficient mold clamping force is achieved, nozzle 172 isactuated to inject molten plastic into mold cavity 170. When the desiredquantity of resin has been injected into the mold cavity, the nozzle isclosed. After a short time delay, generally 1-5 seconds, the gas rod isopened and pressurized gas flows into the hot polymer core. The timedelay allows the plastic pressure to decrease from its injectionpressure before the gas orifice is opened. This helps prevent plasticback flow into the gas nozzle. If desired, it is possible to begin thegas injection before the plastic injection is completed. In such a case,the gas pressure must be sufficiently high to prevent plastic back flowinto the gas orifice. When injected, the gas takes the path of leastresistance through the plastic.

After the resin has been injected into cavity 170, the nozzles 176 and178 are actuated to inject plastic resin into cavity 174 with the gasbeing injected thereafter forming two hollow spaces 180 and 182 in themolded article. Hollow space 180 communicates with nozzle assembly 176while hollow space 182 communicates with nozzle assembly 178. A solidzone of plastic 184 is formed at the interface between the two hollowspaces 180 and 182.

The mold closure force necessary to resist the plastic injectionpressure and hold the mold cavities closed is less if the two cavitiesare injected sequentially, i.e. one at a time, rather than injectingboth cavities simultaneously. As a result, the process cycle time isreduced since less time is required to reach the necessary mold closureforce. Additionally, lighter weight mold components can be used and/orless material used in the mold assembly reducing the cost of materialand the size of the hydraulic system required for moving the mold halvesand developing the closure force.

During the molding process, the nozzle 172 is opened while the plasticaccumulator piston is moved, injecting the plastic into the mold cavity.Once the piston has moved a predetermined distance, supplying thedesired shot to cavity 170, the accumulator piston is stopped and thenozzle rod move to the closed position in nozzle assembly 172. Afterwhich, the nozzle rods in nozzle assemblies 176 and 178 are moved to theopen position and the accumulator piston is again actuated to supplyplastic to cavity 174.

With sequentially injection, the nozzle assembly 172 is operatedindependently of nozzles assemblies 176 and 178, ensuring that theproper shot size is delivered to cavity 170. If the cavities weresimultaneously filled, the relative proportions of resin injected byeach nozzle would be affected by pressure differentials between thenozzles, localized hot or cold spots, etc. As a result, the shot sizeinto each mold could not be precisely controlled. This would also occurif one nozzle was used to feed multiple cavities through a runnersystem.

Furthermore, with the process of this invention, the gas injection intoeach cavity is also independently controlled rather than utilizing asingle control system to simultaneously inject gas into both cavities.This is particularly important with respect to the gas injection sincethe gas will form a hollow passage through the molten plastic followingthe path of least resistance. With sequential gas injection, the gaspath through each mold cavity is independently controlled. Withsimultaneous injection, more gas may flow to one mold cavity thandesired while another cavity does not receive sufficient gas.

The solid plastic block 184 between the two hollow spaces 180 and 182 isuseful to improve the strength of the polymer weld line occurring at thejunction of the two resin shots from each nozzle assembly. If such ablock is not desired, only one nozzle can be used for the gas injectionin which case a single hollow space will be created in the moldedarticle communicating with the nozzle assembly that used to inject thegas. In a typical molding operation, only one nozzle is used to injectthe gas into a mold cavity.

It is to be understood that the invention is not limited to the exactconstruction or method illustrated and described above, but that variouschanges and modifications may be made without departing from the spiritand scope of the invention as defined in the following claims.

We claim:
 1. A combination plastic and gas injection nozzle assembly,comprising:a hollow nozzle body having a longitudinal axis and forming aresin flow passage therein and a resin outlet at one end of said body;means coaxially extending in said body for closing said resin outlet,said closure means being movable between positions opening and closingsaid resin outlet; first moving means coaxial with said body for movingsaid closure means; means forming a coaxial gas passage within said bodyfor a pressurized gas, said gas passage terminating in a gas orifice insaid closure means; a gas valve member movable between positions openingand closing said gas orifice; second moving means coaxial with said bodyfor moving said gas valve member, said second moving means forming a gassupply channel to supply gas to said gas passage, said gas supplychannel extending coaxially with said nozzle assembly, said first movingmeans including a first actuating cylinder having a piston rod coupledto said closure means, said piston rod being hollow with said gas supplychannel extending therethrough, said second moving means comprising asecond actuating cylinder having a hollow piston rod forming said gassupply channel therein.
 2. The nozzle assembly of claim 1 wherein saidsecond moving means is carried by said first moving means for movementalong with said closure means.
 3. The nozzle assembly of claim 1 furthercomprising pressurized gas supply means, said gas supply means includinga pressurizing pump selectively connectable to said gas supply channelto provide pressurized gas to said channel and an exhaust pumpselectively connectable to said gas supply channel for evacuating gasfrom said gas supply channel.
 4. The nozzle assembly of claim 1 whereinthe coupling of said piston rod with said closing means is sealed with astatic seal member.
 5. The nozzle assembly of claim 1 wherein saidsecond cylinder is coupled to the piston rod of said first cylinder formovement of said second cylinder along with said first cylinder pistonrod.
 6. The nozzle assembly of claim 5 wherein the coupling of saidsecond cylinder to said first cylinder piston rod is sealed by a staticseal member.
 7. A combination plastic and gas injection nozzle assembly,comprising:a hollow nozzle body having a longitudinal axis and forming aresin flow passage therein and a resin outlet at one end of said body; aclosure member for said resin outlet movable between positions openingand closing said outlet, said closure member forming a gas flow passageand having a gas outlet orifice, said gas outlet orifice beingpositioned at said resin outlet when said closure member is in saidclosed position; first moving means for moving said closure memberbetween said opened and closed positions, said first moving means beingcoaxial with said nozzle body; a gas valve member movable betweenpositions opening and closing said gas orifice; second moving means formoving said gas valve member between said opened and closed positions,said second moving means being coaxial with said nozzle body; and meansfor supplying gas to said gas flow passage, said gas supply meansincluding said gas valve member which comprises a hollow gas rod axiallydisposed in said nozzle assembly and having means for venting theinterior of said gas rod into said gas flow passage, said gas valvemember including a solid tip at the end of said hollow gas rod forclosing said gas outlet orifice, and said second moving means includingan actuating cylinder coupled to said hollow gas rod for moving said gasrod and tip axially, said closure member comprising an axially elongatednozzle rod, the interior of which forms said gas flow passage and intowhich said gas rod extends, said nozzle rod forming a valve seat at saidgas outlet orifice for engagement by said gas rod tip when in positionclosing said gas outlet orifice, said first moving means including anozzle rod actuating cylinder having a piston rod coupled to said nozzlerod for moving said nozzle rod axially, and said gas rod extendingaxially through said nozzle rod actuating cylinder.
 8. The nozzleassembly of claim 7 wherein said gas rod actuating cylinder is carriedby the piston rod of said nozzle rod actuating cylinder for movementtherewith to retain said tip in the position closing said gas outletorifice as said nozzle rod is moved.